This invention relates to the field of integrated circuit fabrication. More particularly, this invention relates to heat dissipation in packaged integrated circuits.
Integrated circuits are typically packaged prior to use, so that they are both easier to handle, and so that they are less susceptible to damage from handling and environment. Thus, integrated circuit packages provide a valuable function. However, with the benefits of the package also come certain problems. For example, localized areas of electrical activity within an integrated circuit tend to produce thermal energy as a result of electron motion. Unless the thermal energy is dissipated, it tends to build up within the localized areas and is expressed as a temperature increase in the integrated circuit. If the thermal energy builds up to sufficient degree, the temperature can increase to a point where the delicate structures of the integrated circuit either malfunction or are permanently damaged.
When the integrated circuit is open to the environment, there tends to be good thermal transport from the integrated circuit, primarily by convection with the surrounding atmosphere. However, when the integrated circuit is packaged, as is typically desired, the package materials tend to inhibit the dissipation of the thermal energy from the integrated circuit to the surrounding environment. Although the package materials are able to dissipate a certain amount of thermal energy via conductance through the package materials, this amount of thermal energy dissipation often tends to be inadequate. The reason for this is that the materials of the integrated circuit package tend to be primarily selected for other properties, such as the protection that they will provide to the integrated circuit, rather than for heat conductance properties.
For example, one type of packaging material that is commonly used is one or more of a variety of molded thermoplastic resins, commonly called plastic packages. Plastic packages are often preferred for certain types of integrated circuits, such as wire bonded integrated circuits, because of their versatility and ease of formation. For example, the plastic tends to easily flow in and around the wires that electrically connect the wire bonded integrated circuit to the electrical connections of the packaging, providing both mechanical support and electrical insulation to the wires. Unfortunately, the plastic tends to be a very poor thermal conductor. Unfortunately, many modifications that could be made to the plastic itself that would improve the thermal conductivity of the plastic would also increase the electrical conductance or capacitance of the plastic, and would thus be counter productive.
To overcome this tendency for thermal energy to build up in parts of the plastic molded integrated circuit package, additional structures have been added to the package, which additional structures are generally referred to as heat spreaders. The heat spreader is typically a structure formed of a material having a thermal conductivity that is at least somewhat greater than the plastic that is used to encapsulate the integrated circuit, such as metal. The heat spreader typically has the general form of a plate that is disposed over the integrated circuit and extends down into the plastic encapsulant to contact the package substrate. In this manner, the plastic encapsulant helps retain the heat spreader as part of the integrated circuit package. However, the heat spreader should not make contact with the wires that electrically connect the integrated circuit to the electrical connections of the packaging, so that electrical shorting of the wires does not occur.
The package is typically formed by disposing the heat spreader over the wire bonded integrated circuit within a package mold form, and injecting the plastic into the mold form, encapsulating the integrated circuit and portions of the heat spreader. Because the heat spreader is basically a solid plate, and resides over the integrated circuit, the plastic is preferably injected into the mold form from the sides of the mold form, in what is called a side gated injection.
When the packaged integrated circuit is relatively small, a side gated injection works well enough. However, if the packaged integrated circuit is relatively large, then the plastic injected during a side gated process may not distribute properly throughout the package. In addition, molding compound injected from the sides tends to move the wires somewhat as it flows by them in a condition typically referred to as wire sweep. When wire sweep is severe enough, the wires may actually touch each other, thereby electrically shorting the signals that they are to carry.
One method of overcoming these problems would be to inject the plastic from another point in the mold form, such as from the top and center of the mold form, in a top gated process. Unfortunately, the heat spreader interferes dramatically with a top gated injection, because it blocks the flow of the plastic that is injected at the top of the mold form.
What is needed, therefore, is a system by which an integrated circuit may be more uniformly encapsulated in a plastic injection packaging process.
The above and other needs are met by a heat spreader for use with an integrated circuit in a package, where the heat spreader is formed as a plate having a centrally disposed aperture with a diameter that is smaller than a minimum diameter of the integrated circuit. The heat spreader has an overall diameter that is no greater than a minimum diameter of the package. In this manner, the aperture in the heat spreader allows the plastic injected through a top gated mold form to pass through the heat spreader and more uniformly encapsulate the integrated circuit. In addition, because the molding compound is flowing radially outward from the top center of the package, the flow of the molding compound does not sweep the wires into one another.
In various preferred embodiments of the invention, the heat spreader is between about twenty millimeters and about fifty millimeters square, and most preferably about thirty-five millimeters square. The aperture in the heat spreader is preferably between about one millimeter and about ten millimeters in diameter, and most preferably about two millimeters in diameter. Preferably, the heat spreader is between about one tenth millimeters and about five tenths millimeters in thickness, and most preferably about three tenths millimeters in thickness. The heat spreader is preferably made of copper and is nickel plated with a first surface with black oxide treatment.
According to another aspect of the invention there is described an integrated circuit package. A package substrate for receiving an integrated circuit is provided, where the package substrate has electrical contacts. An integrated circuit is disposed on the package substrate, where the integrated circuit also has electrical contacts. Wires are electrically connected by a first end to the electrical contacts on the integrated circuit and are electrically connected by a second end to the electrical contacts on the package substrate. The integrated circuit and the wires reside within a first level. A heat spreader with a centrally disposed aperture with a diameter that is no greater than a minimum diameter of the integrated circuit overlies and surrounds the integrated circuit. A molding compound encapsulates the integrated circuit and portions of the heat spreader to the package substrate.
According to yet another aspect of the invention, there is described a method of packaging an integrated circuit. The integrated circuit is attached to a package substrate having electrical contacts, thereby forming a package subassembly. Wires are wire bonded from the electrical contacts on the package substrate to electrical contacts on the integrated circuit. A heat spreader having a centrally disposed aperture having a diameter that is no greater than a minimum diameter of the integrated circuit, and at least a portion of the package subassembly are placed within a mold cavity. The heat spreader is disposed over but not touching the wires of the package subassembly. The heat spreader is disposed between the package subassembly and a top gate of the mold cavity. A plastic is injected through the top gate in the mold cavity, and the plastic thereby flows through the centrally disposed aperture to encapsulate the integrated circuit and portions of the heat spreader to the package substrate.